Converting 7199s to 6GH8A

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Converting 7199s to 6GH8A

Postby joeztech » Tue Aug 19, 2014 8:16 am

I saw the comments about biasing the heater circuit of a 6GH8A or 7199 with relation to the cathode bias. I have a stock ST-70 with 7199s that test with weak emission. I have a stock of (51) 6GH8A tubes to select from and would like to convert to those. Aside from making some cuts to traces under the board and reconnecting to the appropriate pins plus the heater biasing issue, are there any other component changes that are needed when doing the conversion?

Thanks in advance.
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Re: Converting 7199s to 6GH8A

Postby 20to20 » Tue Aug 19, 2014 8:23 am

To go to the 6GH8 there are no heater biasing issues, you can just keep it stock. And there are adapter sockets being sold (or were), on EB, so you can delay the board cuts until you decide if it's worth the trouble to make a permanent change.

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Re: Converting 7199s to 6GH8A

Postby joeztech » Tue Aug 19, 2014 11:11 am

20to20;

OK Thanks. I also saw the adapters on one of the mfg. websites recently too.

Joe
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Re: Converting 7199s to 6GH8A

Postby joeztech » Tue Aug 26, 2014 2:51 pm

I went through my stock of 6GH8As and found about 7 that have good emission and no shorts. There were a few that came somewhat close to these, but which I rejected because the two sections had emission levels that did not match. I went ahead and made the PC board cuts and added a jumpers to complete the proper connections to the tube contacts. I did a recheck the next day using an ohm meter to make sure all components in the signal path and B+ connected properly to the new pin numbers.

I noticed that the 6GH8A implementation in the Stereo 70 Series 2 uses 0.68mFd coupling capacitors to the signal grids of the two output tubes instead of the original 0.1mFd value. Also they changed the negative feedback resistor from the output transformer secondary from a 1K ohm to a 680 ohm 1W resistor. They also made some changes to the 6GH8A pentode signal grid circuit by adding a coupling capacitor and a roll-off circuit to change the input response curve. The feedback resistor was moved from the 16 ohm output transformer secondary tap to the 8 ohm secondary tap. For now I do not see any need to change this aspect of the circuitry but might try that later on.

Right now I am waiting on resistors and electrolytics for the PC board and power supply to upgrade the voltage rating of the power supply and get the resistors of the PC board back to their correct values. The first order may be here tomorrow. The new output transformer from Heyboer Transformer Co. is on its way and should be here later this week.

Joe
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